The present invention relates to an extruding device for extruding plastic tubing, the device comprised of: an extruder with an outlet; a cooling device connected to the outlet of the extruder with a water-cooled calibrating sleeve within the cooling device for pulling the extruded plastic tubing against the inner wall surface of the calibrating sleeve; a circulating cooling element in the form of a vacuum tank; and a measuring device for measuring the wall thickness of the plastic tubing having one or more ultrasonic measuring heads radially oriented relative to the plastic tubing.
An extruding device of the aforementioned kind is known from German Offenlegungsschrift 39 36 496. In this known device the calibrating sleeve is provided with separate cooling medium channels for guiding therethrough a cooling medium, preferably water. At the end of the calibrating sleeve a measuring device for measuring the wall thickness of the tubing is provided and includes respective measuring heads. The device further comprises an annular recess surrounding the tubing to be measured and into which water is introduced for effecting the transmission of ultrasonic waves between the measuring heads and the tubing to be measured whereby the measuring heads are radially arranged within the water-filled recess.
The known extruding device has the disadvantage that the calibrating sleeve is of a relatively complicated design due to the separate cooling medium channels. Furthermore, a separate feed line must be connected to the calibrating sleeve in order to introduce the cooling medium. A further technical disadvantage is that the ultrasonic coupling of the measuring heads with the plastic tubing to be measured is insufficient. Since the plastic tubing passes through the measuring device during the measurement and also passes the inner side of the water-filled annular recess, sealing of the annular recess is almost impossible to control. This is so because the plastic tubing is still deformable since it is still soft and thus variable at its outer diameter, for example, by changes of the vacuum as well as due to the vacuum present within the chamber so that water loss within the area of the annular recess cannot be prevented. However, these water losses affect the accuracy of the measurements considerably. An additional disadvantage is that the height of the water column between tubing and measuring head depends on the wall thickness of the plastic tubing to be measured so that for a change in the wall thickness the water column must be adapted. This, however, is almost impossible due to the design-based dimensions of the annular recess within the measuring device.
It is therefore an object of the present invention to provide an extruding device of the aforementioned kind with which the accuracy of the measurements is improved, while at the same time the extruding device must be simplified in its design and improved with respect to its operational reliability.